Mark Gill asked: >>>>>I have a question for the group on the effects of temperature rating and deterioration. If a pcb is rated X degrees C, and the sizing of the power traces on an inner layer is such that the pcb temperature is now (X-1) degrees C,
What effect does this have on the longevity of the pcb?<<<<<< A while back, I was a test engineer in the PCB category. Generally, the substrate material (and in fact, most thermoplastic and thermoset materials) obtained temperature ratings based on a process outlined in the UL 746 series of standards: The manufacturer of the substrate estimated a tempertature rating for the material at a minimum thickness. Based on that temperature rating, three oven aging temperatures roughly 10 degrees C apart were assigned to simulate long term aging. For example, if the hoped-for temp rating was 110 degrees C, the board substrates were placed in three separate ovens at about 150, 160 and 170. Certain physical characteristics (electrical and mechanical) of the materials were first measured on as-received samples - including dielectric withstand breakdown point, tensile strength, and others. After several months of aging at the higher temperatures, samples were retested. If the physical characteristics had not degraded to 50% of the original values, the test continued. If they had, the test stopped at that aging temp, and the time was recorded. Of course, the higher temp samples usually reached the 50% values first, then the middle, then the lower. At the conclusion of the test - when all three temperature samples reached 50% breakdown, curves were drawn based on time and temperature, then extrapolated out to the 20 year mark. If, at the 20 year mark, the time temperature curve was within a few degrees of the expected temperature rating, the material then received the requested rating. Sometimes the rating was adjusted upwards or downwards accordingly, other times the test was re-run at newly calibrated aging temperatures. It is a lengthy and expensive process. I don't know how much this helps you, but the overall philosophy on temperature rating of laminates is that after operating for 20 years at the rated temperature, the critical physical properties of the material should not have degraded to below 50% of the unaged sample. Greg Galluccio www.productapprovals.com <A HREF="www.soundsurfr.com"> </A>
